The record made of the information in the recording layer commonly appears in the form of surface irregularities on the order of one micron. The operations for the production of optical discs, whether prerecorded or blank, are performed in a dustfree atmosphere with numerous precautions to prevent any deterioration of the carrier or of the sensitive layer. It is while the disc is being used by a user that deteriorations may occur, as well as while it is being stored. Consequently, it is desirable to protect the recording surface against anything which could cause damage to the same: dust, fingerprints, placing the heat-sensitive layer in contact with objects able to spoil or scratch the same. The system for protection of an optical disc should also permit easy and frequent handling by a user.
It is known that sensitive layers used to record data may be deposited on a rigid carrier and protected by means of a rigid plate of a thickness close to one millimeter maintained at a particular distance from the carrier by means of spacers. The major disadvantage of this structure derives from the assembling of the four elements forming the same: the rigid carrier, the rigid protective plate and the two circular spacers. In particular, the peripheral spacer raises a serious problem regarding centering since it has no contact surface at all with the centering spindle, in contrast with the three other elements. The speed of revolution of an optical disc being on the order of 1500 r.p.m., any faulty balancing causes the production of appreciable centrifugal forces and the occurrence of vibrations prejudicial to a correct operation of the system.
Consequently, it has been attempted to avoid the necessity of spacers and to simplify the assembly of the protected disc by means of a preformed rigid protective cover or lid bonded to a rigid carrier transparent to the radiation, the sensitive layer being deposited on this carrier. Although this resolves the disc balancing problem, another problem arises in view of recording and playback through the rigid carrier. In effect, the variations in thickness occurring in this approach on one disc, or from one disc to another, result in optical aberrations modify the energy distribution within the optical spot focussed on the sensitive layer thereby impairing the quality or reproducibility of the recording. On the other hand, sensitive structures of the triple layer type cannot be recorded acceptably through the substrate.
In order to overcome these shortcomings, the present invention makes an optical disc from a rigid carrier which is deposited on a sensitive layer, and from a thin element providing access for the playback beam, the rigid carrier and/or the transparent cover being preformed in such a manner as to delimit an annular chamber above the sensitive layer without the necessity of inset spacers.